I don't have a helmet cover, but after my ride yesterday I may get one.
Another factor in whether to use a helmet cover (with or without balaclava) is how much hair you have. I myself am follicularly challenged, so on cold days I wear a helmet liner with a balaclava pulled up over it (the helmet topping everything off, of course). It was 23F during my ride yesterday and my head got chilled during the downhill bits. Where I live it doesn't rain much--just snow or sun, and that windchill factor can be a bear.